New method isFoo() exists for union fields only and returns true if foo is the active union field.

Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

Complete C++ .capnp parser.  Also, add a pretty-printing function for arbitrary Cap'n Proto objects which is like the existing stringification but uses whitespace (newlines an indentation) to make the structures more readable.

Use consistent naming style for structs that exist purely to specialize template aliases or functions.

Improve const-correctness of readers and builders.  We enforce transitive constness, meaning you should not be able to construct a mutable Builder from a const Builder (you can always convert them to Readers instead).

Gracefully handle the case where the schema contains types from a newer version of Cap'n Proto that we don't know about yet.

Implement ability to set() struct and list fields by deep-copying from a corresponding Reader.  This copy requires no schema knowledge.

Mass rename 'reference' -> 'pointer'.  Been meaning to do this for a while -- all the documentation talks about 'pointers', not 'references'.

Revamp dynamic struct getters/setters to not distinguish unions.  Add converience versions that take string names.

Dynamic API mostly done...  not tested.  This commit actually starts embedding the schema data in the generated code.